Note that the max HR recorded was 16bpm higher than your original post stated was the highest you had seen while riding.

You've tested for your LTHR. I'd be inclined to use that as a guide, and do some criss-cross intervals - those described by Allegheny Jet as "over/unders" but using your LTHR rather than speed as the reference point. So, three minutes at 90% of LTHR followed by two minutes at above LTHR - not as hard as you can go, but nearly - repeated twice for a ten minute interval. Then go very easy for six minutes to recover, and repeat. This will, over time, bump up the power you can produce at threshold. As you get fitter you can increase the severity of the interval by altering the proprtions of time you spend above and below LTHR.

But give yourself time to recover. A couple of interval sessions like this per week is going to be enough. The rest of the time, ride normally. This is supposed to be fun, after all.